The short wavelength blue semiconductor laser was recently introduced into the market for practical use, and BD (blue-ray disc) having such a larger storage capacity as 25 GB or more is now prevailing over DVD (digital versatile disc) with the storage capacity of 4.7 GB. The technical merits of BD are; the blue semiconductor laser having the wavelength of 405 nm is used in place of the conventional 650-nm red semiconductor laser to make a laser spot on an optical disc smaller, and NA (Numeral Aperture) of an object lens used to narrow down the laser spot on the optical disc, which is conventionally “0.6” in DVD, is increased to “0.85” in BD. The synergy effect of these technical merits has accomplished such a large storage capacity.
As a result of NA of the object lens thus increased, the problem of spherical aberration, which is a difference between an ideal focal point and an actual focal point of laser transmitting through an optical system, becomes markedly noticeable. The spherical aberration, which is generated by the thickness error of a cover layer of the optical disc, is in proportion to the fourth power of NA, meaning that the problem of spherical aberration may become more noticeable depending on the structure of DVD. In optical discs which require a very high NA such as BD, a standardized cover layer thickness is 0.1 mm, and a thickness error tolerance is equal to 10 μm or less. Therefore, the spherical aberration is variable in different optical discs, making it necessary to correct the spherical aberration of each optical disc independently in an optical pickup. Conventionally, the spherical aberration is mostly corrected by mechanically operating a spherical aberration correction device including a spherical aberration correction lens. The spherical aberration correction device is provided between a laser light source and an object lens to be movable by a lens drive unit equipped with, for example, a stepping motor.
This section describes the performance of the spherical aberration correction device in responsiveness to temperature variation. For price reduction, a plastic lens is most often used in the spherical aberration correction device. The plastic lens, however, is likely to decrease its index of refraction with temperature rise, and overly low index of refraction is detrimental to recording and reproduction qualities. In order to control the thickness error of the cover layer and minimize the influences from temperature variation to reliably accomplish good recording and reproduction qualities, it is necessary to constantly move the spherical aberration correction device to an appropriate position at the time. Therefore, conventional optical disc recording and reproduction apparatuses usually obtain an ambient temperature using a temperature sensor and move the spherical aberration correction device to an appropriate position corresponding to the obtained ambient temperature before starting to read signals. However, it invites cost increase to provide the temperature sensor. To dispense with the temperature sensor, it is preferable to measure the spherical aberration by some kind of method and adjust the position where the spherical aberration correction device should be positioned based on the measured spherical aberration. A conventional well-known technique, though only applicable when signals can be read from an optical disc, is to find a position of the spherical aberration correction device where the best signal quality is obtained because of a close relationship between the spherical aberration and the quality of signals read from the optical disc.
To read signals from an optical disc, it is necessary to adjust a laser focal point on the optical disc by driving an object lens, which is called a focus operation. The focus operation needs a focus error signal. The spherical aberration is a bottleneck in generating the focus error signal in good condition. Therefore, the spherical aberration correction device should be positionally adjusted before signals are read from the optical disc to lessen the spherical aberration to such an extent that does not affect the focus operation. When the position the spherical aberration correction device is thus searched based on the signal quality, however, it is not possible to correct the spherical aberration in a stage where signals have not been read from the optical disc. There are the prior arts disclosed in the Patent Document 1 to solve the technical problem; detecting the spherical aberration using a focus balance difference which is a difference between absolute values of positive and negative peaks of the focus error signal (hereinafter, called prior art 1), and detecting the spherical aberration using an interval between positions where the positive and negative peaks are detected (hereinafter, called prior art 2).